| Immune Basics
Getting sick is often a sign that your body is responding to invaders
from the outside. For example, the pus around a splinter that gets
infected is a sign of immunological effort - white blood cells building
up to fight an infection. The same is true when your body’s immune
system responds to HIV. Your immune system is a complex network
of cells and organs that work together to defend your body from
viruses, bacteria, fungi, and protozoa. There are two types of immune
response: innate and acquired. The innate response starts very quickly
after infection and does not depend on recognizing the specific
type of foreign substance that is present in your body. Natural
killer cells are a component of innate immunity. Acquired immune
response starts more slowly, but it also lasts longer. In addition,
the acquired response is specific to certain foreign substances
and it can "remember" past infections. This memory allows the immune
system to react quickly if you are ever re-infected with the same
pathogen.
Two important components of acquired immunity are antigens and
antigen-presenting cells. An antigen is a foreign substance, usually
a protein, which can harm your body and cause your immune system
to respond. Antigen-presenting cells are already in your body and
digest these intruders and display the antigen on their surface.
Antigen-presenting cells then look for other immune cells that respond
to that specific antigen.
One type of cell that antigen-presenting cells look for is called
a B cell. Each B cell recognizes one and only one type of antigen.
When the antigen-presenting cells find B cells that recognize the
antigen, those B cells become active and begin to reproduce. Most
of the new B cells release antibodies. Antibodies play several roles,
including marking foreign matter and infected cells for destruction
by other parts of the immune system. (The standard test for HIV
does not actually look for the presence of HIV but antibodies that
your body has produced to fight HIV. It usually takes about a month
or six weeks for your body to produce enough antibodies to register
on an HIV test, a point known as seroconversion.) Once the infection
is under control, these antibody-creating B cells are no longer
needed and they die. However when B cells reproduce, they also produce
"memory B cells" that can live for years. If the same antigen is
encountered again, the memory B cells activate and reproduce more
quickly than the first time.
Antigen-presenting cells can also look for T cells. Like B cells,
T cells respond to only one type of antigen. Again, once the right
T cells are found, they become active and reproduce. But instead
of producing antibodies, T cells release cytokines. Cytokines direct
immune cells to attack a foreign substance. Furthermore, there are
two types of T cells: CD4 T cells and CD8 T cells. CD4 T cells are
also called "helper T cells" because they help other parts of the
immune system respond to infection. Specifically, CD4 T cells stimulate
B cells to respond and increase the production of CD8 T cells. Although
CD8 T cells have many roles, one of the most important is to kill
the cells that have become infected by an antigen. These CD8 cells
are called "killer cells" or "cytotoxic T lymphocytes" (CTLs). As
with B cells, once the infection is under control, many of these
T cells are no longer needed and they die. However, some T cells
become memory cells, which can respond quickly if the antigen is
encountered again.
HIV, however, is more formidable an enemy to the immune system
than most. It reproduces by installing itself in the CD4 cells that
coordinate the rest of the immune response. Once HIV is inside them,
these T-cells become virus factories, producing thousands of viral
particles and then dying off. A person infected with HIV produces
millions and millions of new viral particles every day. This is
the process of viral replication.
The more CD4 cells HIV kills off, the more powerless your immune
system becomes to fight off other bacteria, fungi, parasites, and
viruses. It is often these germs and bugs - some of which may have
been inside your body and controlled by your immune system since
childhood — that make you sick in the course of HIV illness. Because
they take advantage of the opportunity provided by your weakened
defenses, many of the most serious of these illnesses are known
as opportunistic infections, or OIs. Many of these, and ways to
treat them, are discussed in the fact
sheets produced by GMHC's Treatment Education and Adherence
Program.
What can you do? Deciding whether
to treat HIV or not is a tough decision. Though it can take
years and years before HIV will destroy your immune system without
treatment, for 95% of HIV-positive people the virus will eventually
win unless you take medications to help.
© 2003 Gay Men's Health Crisis |
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